Acousto-optic devices use sound waves to diffract light. In a typical device of this sort, a transducer, such as a piezoelectric transducer, is attached to an acousto-optic medium, typically a suitable transparent crystal or glass. The transducer is driven by an electrical signal to vibrate at a certain frequency, and thus creates sound waves in the acousto-optic medium. The expansion and compression of the acousto-optic medium due to the sound waves modulate the local index of refraction and thus create a grating structure within the medium, with a period determined by the frequency of the drive signal. A beam of light that is incident on this grating will thus be diffracted as it passes through the device.
Various types of acousto-optic devices are known in the art. Acousto-optic deflectors, for example, use the diffraction of the incident beam to steer the angle of the output beam. The angle of deflection of the output beam depends on the period of the grating structure in the acousto-optic material and may thus be adjusted by appropriately varying the drive signal frequency.
Acousto-optic deflectors may be driven with a multi-frequency drive signal in order to diffract the incident beam into multiple output beams at different, respective angles. Further details of this sort of multi-frequency drive are described, for example, by Hecht in “Multifrequency Acoustooptic Diffraction,” IEEE Transactions on Sonics and Ultrasonics SU-24, pages 7-18 (1977), which is incorporated herein by reference; and by Antonov et al. in “Efficient Multiple-Beam Bragg Acoustooptic Diffraction with Phase Optimization of a Multifrequency Acoustic Wave,” Technical Physics 52:8, pages 1053-1060 (2007), which is likewise incorporated herein by reference.
Acousto-optic devices with multiple output beams have also been described in the patent literature. For example, U.S. Pat. No. 5,890,789 describes a multi-beam emitting device, which splits a light beam emitted from a light source into a plurality of beams using an optical waveguide-type acousto-optic element or the like, driven with a plurality of electric signals with different frequencies. As another example, U.S. Patent Application Publication 2009/0073544 describes a device for the optical splitting and modulation of monochromatic coherent electromagnetic radiation, in which an acousto-optical element splits the beam generated by a beam source into a number of partial beams. An acousto-optical modulator disposed downstream of the acousto-optical element is fed the split partial beams and driven with additional high-frequency electrical signals.
As still another example, U.S. Pat. No. 5,255,257 describes an electronic circuit that is said to allow an acousto-optic deflector to be used in multi-frequency mode at high power levels with a minimum amount of intermodulation between each frequency. Reduction of interference between multiple separate signal frequencies is achieved by precise control of the individual phasing of each separate frequency relative to a common reference frequency. The relative phase of each frequency is also controlled so that a low maximum power is achieved for the combined signal that is presented to the acousto-optic deflector without decreasing the overall average power of the multiple signal frequencies.